Mississippi State University Weather

Weather Radar: How Does It Work?

Doppler radar is one of the most important tools a meteorologist uses to determine what is happening in real-time where he or she is forecasting. Most people are familiar with the green, yellow, and red pictures that radar generates because they see them on TV, but radar actually gives a meteorologist more information about the atmosphere than a simple image of rain.

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Image Credit: Hong Kong Observatory
http://www.hko.gov.hk/wservice/tsheet/radmet1.jpg

 

A radar sends out a beam of radiation into the atmosphere. If there is rain falling, the radiation hits that rain and bounces back to the radar. Depending on how much energy is reflected back to the radar and how fast that radiation is bounced back, we can determine where and how heavy the rain is.

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Image Created Using Gibson Ridge Software

The radar then sends this information through a computer that gives us the pretty green, yellow and red maps you see on TV when we are tracking storms and rain. Typically, the heavier the rain, the warmer the color. So, green usually means light rain, yellow means moderate rain, and red means heavy rain or hail.

Radars can also measure winds, but that is a bit more difficult to interpret. A radar uses the same physical principles that explain why an ambulance siren sounds higher pitched as it is moving towards you and lower pitched as it moves away to measure winds inside of thunderstorms. By determining the frequency of the radiation reflected back to the radar site, the radar can determine whether the rain is moving towards or away from the radar, and how fast. From this information, a meteorologist can infer wind speed and direction inside a storm.

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Image Created Using Gibson Ridge Software

This is the image a meteorologist would look at to determine wind speed and direction inside a thunderstorm. The green colors indicate winds moving towards the radar, and the red colors indicate winds moving away from the radar. The brighter the color is, the faster the winds are. That yellow box is a Severe Thunderstorm Warning that was issued because meteorologists used this image to estimate the storm was producing winds of 60 miles per hour.

One of the biggest limitations with radar is that the radar can’t see what is happening at the surface because of the curve of the earth. The radar beam, which is emitted in a straight line, overshoots the surface because the earth’s surface is curved.

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Image Courtesy National Weather Service
http://www.erh.noaa.gov/gsp/localdat/cases/2006/14May_CabarrusTornado/curvature.gif

Because of this limitation, good forecasters take what radar is showing and add it as a piece of the puzzle rather than assume it is correct. In severe weather situations, storm spotter reports can help a forecaster confirm that what he or she is seeing on radar is making an impact on the surface.  ~ Alex Puckett